Infant jaundice, or hyperbilirubinemia, is a significant clinical problem, occuring in about 5% of full-term infants. The syndrome is the direct result of increased bilirubin levels in the infant body. Adults also have problems with jaundice, but they are generally not as serious or as widespread because most adults are capable of conjugating excess bilirubin with sugars and clearing this toxin from the body. This detoxification mechanism is not fully developed in neonates. Nevertheless, some adults, such as those who have hepatitis or obstructions to bile flow, are subject to jaundice as well.
Various treatments have been suggested for both infant and adult jaundice when these problems occur. These treatments include phototherapy, exchange transfusions, extracorporeal filtration systems, and drugs which induce an efficient clearance system. None of these treatments is simple to administer or effective without negative side reactions, including risk of injury or death. If the jaundice is not promptly treated, serious damage to the nervous system can result, especially in infants, as the elevated amounts of bilirubin act as a neurotoxin, and the blood/brain barrier in infants is incompletely developed. Also, the foregoing treatments are administered after the fact--i.e., after the jaundice has already appeared.
In neonates, the visible signs of the disorder manifest themselves usually at about 72 hours after birth, often after the infant has left the hospital or birth center. Thus, the signs may appear when the baby is no longer under the observation of trained medical personnel. In order to minimize the organic and neurological damage caused by the elevated bilirubin levels, therefore, it is advantageous to intervene before control over treatment has been lost, which is often before the signs actually appear.
One aspect of effective intervention is the identification of individuals at risk for developing this syndrome. Because, in order to eliminate as totally as possible the incidence of neonatal jaundice, every infant must be tested, effective prediction requires a simple, noninvasive procedure. Measurement of bilirubin in the blood per se is not a satisfactory approach because accurate prediction of a potential to develop jaundice rests on detection of increased bilirubin production, as opposed to the levels of bilirubin in the blood. Blood bilirubin levels are influenced not only by production, of course, but also by rates of excretion, and hepatic and intestinal uptake.
Excreted CO rate serves as a valid measure of bilirubin production because CO is a byproduct of the oxidation of heme by heme oxygenase and this is the major source of carbon monoxide generated from metabolic processes (Ostrander, C. R. et al. J Lab Clin Med (1982) 100: 745-755).
It has been shown that a simple noninvasive test for CO production can be used to predict, the risk for development of hyperbilirubinemia in infants, and, more importantly, also identify infants who will not have serious jaundice with high accuracy. Smith, D. W. et al. Pediatrics (1985) 75: 278-280, shows a high correlation between positive results in measurement of carbon monoxide concentration in a mixed end-expiratory gas collection--i.e., an "end-tidal sample" (ET.sub.CO) and subsequent development of jaundice. The breath carbon monoxide concentration directly correlates with bilirubin production as measured by the CO excretion rate. This correlation has been shown by, for example, Smith, D. W. et al. J Pediatric Gastroenterology and Nutrition (1984) 3: 77-80. This latter paper showed that ET.sub.CO is directly related to the pulmonary excretion rate of CO, which is known to be a measure of bilirubin production, but which must be measured using a complex breathing apparatus which encases the infant's head. ET.sub.CO, on the other hand, permits the expired gas to be drawn through a simple nasal catheter. Improvements have also been made in the technique for measuring the CO per se, as disclosed in its application to CO measurement in sample tissues, such as blood (Vreman, H. J., et al, Clin Chem (1987) 33: 694-697.
Not only does ET.sub.CO correlate well with bilirubin production, its predictive value in identifying subjects not at risk to develop jaundice is virtually completely successful--i.e., those who show normal ET.sub.CO values are virtually certain not to develop a metabolically serious form of this syndrome. About 60% of those with elevated levels go on, if not treated, to show the signs of jaundice. Because ET.sub.CO is related to hemolysis, it identifies only those who will develop serious jaundice with dangerous medical effects on the patient.
When subjects at risk are identified, they must either be monitored for subsequent treatment or administered a treatment in advance which prevents the onset of serious jaundice. Because heme oxygenase is an essential enzyme for the production of bilirubin, inhibitors of this enzyme have been suggested as chemopreventive agents. The use of tin protoporphyrin has been suggested as such an agent by Drummond, G. S. and Kappas, A. Science (1982) 217: 1250-1252. However, tin, when administered at high levels in most forms is toxic, and administration of this compound carries sufficient inherent hazard that its use as a preventative is subject to considerable hesitation and reluctance. Even if only 40% of the neonates with elevated ET.sub.CO levels do not, in fact, need to be treated with a preventative, the possibility that the infant is being subjected to an unnecessary risk is clearly an inhibition to administering this agent. McDonagh, A., J Photochem Photobiol (1987) 1: 127-133 estimates that 1 mg/kg doses of tin protoporphyrin can be tolerated by humans; this may be an upper limit.
The zinc analog of the tin compound has also been suggested for chemoprevention (Qato, M. K. and Maines, M. D. Biochem J (1985) 226: 51-57, however, its efficacy is controversial as it appeared to be ineffective in the hands of Drummond and Kappas (supra). In fact, neither group demonstrated its effect, shown hereinbelow, on bilirubin production.
Drummond and Kappas (supra) found that zinc protoporphyrin at doses up to 50-fold greater than the effective dose of tin protoporphyrin did not prevent increase in tissue heme oxygenase activity and serum bilirubin levels. While the Qato paper suggests the use of zinc protoporphyrin for prevention of neonatal hyperbilirubinemia, Drummond showed that only at extremely high dosage levels (500 umol/kg) was zinc protoporphyrin able to lower, after a considerable, 7 day, lag period, hepatic heme oxygenase activity in rats, and was unable to prevent the normal postnatal rise in the levels of this enzyme, or the postnatal rise in plasma bilirubin. Tin protoporphyrin in their hands was effective at 10 umol/kg. Qato, however, found that zinc protoporphyrin administered at 40 umol/kg subcutaneously into nonhuman primates was able to lower serum bilirubin levels and to inhibit the activity of heme oxygenase in the liver and spleen. These results appear to conflict with those obtained by Drummond.
Zinc protoporphyrin, like zinc compounds generally, has the advantage of a virtual absence of toxicity as zinc is an essential element which is nearly as abundant as iron. It has an established minimum daily requirement of about 3 mg/day, or about 50 umol/day in neonates. At these levels, zinc complexes are completely safe, and could be used with little hesitation for preventive therapy; the 40% of infants in the high ET.sub.CO group who in fact are not in need of chemoprevention would not be harmed by its administration.
The present invention provides a synthesis of the potentially useful zinc protoporphyrin-related chemoprevention regime with a noninvasive screening procedure to provide a systematic program for prevention of neonatal jaundice. The approach can be applied, if desired, to adult jaundice as well.